262 AYERS ON THE DEVELOPMENT 



formed. This blastoderm was composed of cells resulting, in his opinion, from a total seg- 

 mentation of the egg. It surrounded a central cavity which Metschnikoff considers homol- 

 ogous to the segmentation cavity of Copepods and certain Daphnias, and therefore he 

 gives it as his opinion that this cavity existing in Teleas is a genuine segmentation cavity. 

 At the close of segmentation there lies outside the blastoderm a cluster of round cells 

 which later form about the embryo a membrane analogous to the " amnion " (equivalent 

 to the serosa of authors) of other insects. When this membrane is fully formed the 

 embryo is seen to be a round, laterally compressed body. A median furrow now appears 

 causing a division of the embryo into symmetrical halves, the so-called Keimwiilste. 

 The embryo elongates and becomes kidney-shaped, the furrowed face remaining convex', 

 the opposite one becoming concave. While the embryo still consists of a single" layer of 

 cells, the head region is differentiated, and the continuation of the above mentioned fur- 

 row into this region gives rise to two well-marked head folds (Kopflappen). With the 

 further growth of the embryo its head broadens and its posterior end becomes conspicu- 

 ously narrower. The cells of the dorsal wall become broader and thinner, while those of 

 the ventral wall remain cylindrical and, on the whole, increase in thickness, so that one 

 may now for the first time speak of a germinal band (Keimstreif). With still further devel- 

 opment there is formed on the head a transverse fold, the lateral edges of which are 

 especially well marked and are afterwards converted into the pointed jaws, while the 

 median part of the fold is only faintly indicated and finally disappears altogether. The 

 central cavity now becomes filled with small round cells which are derived from the ven- 

 tral plate and soon form the mesenteron. Invaginations at either end of the embryo form 

 the stomodaeum and proctodaeum. The latter remains unconnected with the mesenteron 

 during the whole of the first larval period. The hind part of the embryo grows rapidly in 

 length, whereby it is considerably narrowed and finally is converted into the long, pointed 

 tail. A muscular system is developed in the embryo, now a larva, and a cuticular covering 

 is secreted about the body. This cuticula is armed with bristles, disposed on both sides 

 the equator of the embryo and they are moved in only one direction by muscles. The 

 embryo now comes out of the " amnion " and feeds on the yolk of the host egg. The ger- 

 minal stripe remains in the larva as an undifferentiated band of cells from which (but 

 only at a much later period) the ventral portion of the nervous system is formed ; whereas 

 the youngest larva is already furnished with a bilobed brain mass. Metschnikoff's 

 endeavors to determine whether an amnion (Deck- or Faltenblatt) was present or wanting 

 proved entirely fruitless. 



Ganin (19) studied the development of several genera of the Pteromalidae, amono- 

 others a species of the genus Teleas. The following is an abstract of his observation" 

 on that species. The author takes exceptions to Metschnikoff's view of the existence 

 of a central cavity in the segmented egg of Teleas and, although not having seen the 

 segmentation of the egg in this species, he still believes that it ought to agree closely 

 with Platygaster, Polynema and Ophioneurus, where the result of segmentation is an 

 outer layer of small cells surrounding a solid mass of large " central cells." He o- es so 

 far as to say that the production of a segmentation cavity in the egg of Teleas as the result 

 of a total segmentation is, at least for him, inconceivable. Ganin claims that Metschnikoff's 

 observation regarding the origin of the mesenteron is also erroneous, since he believes that 



